The invention relates to a wide-angle slip-on viewfinder for analog and digital wide-angle photographs with lenses of different focal lengths on range finder cameras.
The design is similar to that of an optical rangefinder system, as is known for example from DE 100 10 443 B4, and is fitted in so-called rangefinder cameras.
Rangefinder cameras with interchangeable lenses of different focal lengths are usually equipped with a viewfinder system, integrated in the camera, of constant magnification, which system is optimized for a specific lens focal length. If lenses with longer or shorter focal lengths are used, image field delimitation frames, which correspond to the modified recording image field of the lens which was selected in each case or which are adapted to its focal length, of different size must be reflected into the viewfinder for accurate image field selection. The longer the focal length of the lens is, the smaller the recording image field becomes which can be displayed in the viewfinder. Accordingly, the recording image field becomes increasingly larger at shorter focal lengths and can no longer be displayed in the viewfinder as of a specific lens focal length. The constant magnification of the viewfinder system integrated in the rangefinder camera does not permit complete display of the recording image field, with the result that the user can no longer completely see the image field recorded by the lens in the viewfinder.
For this reason, variable viewfinders integrated in the camera with variable focal length are known, which match the image section in the viewfinder to the respective lens focal length. These integrated variable viewfinders are, however, elaborate and of complicated optical design and, on account of the confined installation conditions, often have a poor optical imaging performance. Mask frames for recording image field delimitation are not necessary in these variable viewfinder systems, because the field of view which can be seen in the viewfinder corresponds to the size of the recorded image.
Likewise known are specific viewfinders for wide-angle lenses, the image field of which is adapted to that of the respective lens, and which can be pushed via an adaptor into the hot shoe on the camera. The basic design of such systems, which are often known as Galilean viewfinders, constitutes an inverted Dutch telescope. They have a divergent lens on the object side. This lens simultaneously delimits the field of view and therefore frequently has a shape similar to the recording format. A convergent lens acts as an eyepiece. The field of view is a function of the ratio of the focal lengths of the two lenses.
Furthermore, a slip-on variable viewfinder for wide-angle recordings is known from DE 101 12 611 C1, which viewfinder, however, does not have mask frames for crisp image field delimitation.
Owing to their rigid connection to the camera, slip-on viewfinders have the disadvantage of mostly being suitable only for recordings as of a relatively large object distance, since the optical axes of the lens and of the slip-on viewfinder are arranged at a predetermined angle with respect to one another. Although it is ensured in this manner that the image section recorded by the camera at a relatively large object distance corresponds to the image section displayed in the slip-on viewfinder, parallax errors inevitably occur in the case of image recordings in the near field. The image selection in the slip-on viewfinder then no longer corresponds to the actual image recording of the camera.
The fact that the user must also undertake to set the distance, stop and exposure of the camera also with fitted slip-on viewfinder by looking into the viewfinder on the camera, results in particular design requirements for the slip-on viewfinder. Looking into the camera viewfinder is not to be impeded by the slip-on viewfinder. The distance when switching to or from looking into the slip-on viewfinder is not to be too large. The viewfinder magnification is to be as large as possible so as to be able to detect details effectively. This requires generally a large lens diameter in the front lens. In order to counteract a parallax error, the distance between the optical axes of the slip-on viewfinder and of the lens is to be as small as possible. On the other hand, high speed wide-angle lenses often have a large diameter, and this can impede the field of view of the slip-on viewfinder.
Because of these stipulations, to date only special slip-on viewfinders have been developed as Galilean viewfinders of very simple design for specific focal lengths for the image recording format of the miniature film. With an increase in the number of lenses, the user therefore faces the necessity of always having available an ever increasing number of slip-on viewfinders matching the focal lengths of the lenses used in each case. A mix up in the assignment cannot be ruled out when interchanging lenses and slip-on viewfinders and errors during the image recording are the result.
Increasingly, there has been a desire to use existing lenses on digital cameras. If the sensor size of the digital camera has an image recording format which deviates from the conventional miniature film, the image section displayed in the slip-on viewfinder no longer corresponds to the actual image recorded by the digital camera. If the digital camera has, for example, a sensor size which is smaller by a factor of 1.33 as compared to the miniature film format, a so-called focal length extension by a factor of 1.33 ensues. The slip-on viewfinders matching in each case for the image recording format of the conventional miniature film in this case display an image field which is too large by a factor of 1.33. The image field actually detected by the sensor area in the digital camera is smaller. Errors during the image selection and in the image composition ensue.